JP2008109021A - Semiconductor light emitting element - Google Patents

Abstract

A semiconductor light emitting device capable of extracting light having a high polarization ratio while increasing the amount of light extracted from a light extraction surface.
A semiconductor light emitting device includes a substrate, a nitride semiconductor multilayer structure laminated on the substrate, an anode electrode, a cathode electrode, and a reflective layer. The substrate 2 is made of a single crystal of GaN, and the main surface of the substrate 2 is an m-plane that is a nonpolar surface. The nitride semiconductor multilayer structure 3 includes an active layer 12 that can emit light, and includes a GaN layer, an InGaN layer, and an AlGaN layer. The reflection layer 7 is for reflecting the light traveling in the direction opposite to the light extraction direction A in the light extraction direction A, and is formed on the bottom surface 2a of the substrate 2 which is the surface opposite to the light extraction surface A. Is formed. The reflective layer 7 is made of Al, and is Schottky connected to the bottom surface 2a of the substrate 2 to reflect light.
[Selection] Figure 1

Description

  The present invention relates to a semiconductor light emitting device having a nitride semiconductor multilayer structure.

  2. Description of the Related Art Conventionally, semiconductor light emitting devices having various nitride semiconductor multilayer structures having active layers capable of emitting blue light or the like are known.

  For example, Patent Document 1 discloses a semiconductor light emitting device having a nitride semiconductor multilayer structure in which a plurality of nitride semiconductor layers such as a GaN layer, an InGaN layer constituting an active layer, or an AlGaN layer are laminated on a sapphire substrate. Has been. In this semiconductor light emitting device, a reflective layer made of aluminum nitride is formed on the bottom surface of the substrate.

In the semiconductor light-emitting device of Patent Document 1, by providing a reflective layer on the bottom surface of the substrate, light that travels in the direction opposite to the light extraction direction from the active layer, that is, light traveling toward the substrate side is reflected and extracted from the light extraction direction. It is possible. Thereby, since the light emitted from the opposite side to the light extraction direction can be extracted from the light extraction direction, the amount of light extracted in the light extraction direction can be improved.
Japanese Patent No. 3412563

  However, in the semiconductor light emitting device of Patent Document 1 described above, the amount of light extracted in the light extraction direction can be improved by providing the reflective layer, but the light emitted in the active layer is not polarized, There is a problem that the polarization ratio of the extracted light is low.

  The present invention has been made in order to solve the above-described problems, and an object of the present invention is to provide a semiconductor light emitting device capable of extracting light having a high polarization ratio while increasing the amount of light extracted from the light extraction surface.

  In order to achieve the above object, the invention according to claim 1 is a semiconductor light emitting device having a nitride semiconductor multilayer structure including an active layer capable of emitting light, wherein the main surface of the nitride semiconductor multilayer structure is: A reflection layer for reflecting light to the light extraction surface is provided on the surface opposite to the light extraction surface from which light emitted by the active layer is extracted, which is a substantially nonpolar surface or a substantially semipolar surface. The reflective layer is made of metal, and is a semiconductor light emitting element characterized by being Schottky connected to the formed surface.

  Here, the substantially nonpolar plane is a concept including a nonpolar plane and a plane whose off angle from the plane orientation of the nonpolar plane is within ± 1 °. Further, the substantially semipolar plane is a concept including a semipolar plane and a plane whose off angle from the plane orientation of the semipolar plane is within ± 1 °.

  The invention according to claim 2 is the semiconductor light emitting element according to claim 1, wherein the surface on which the reflective layer is formed is mirror-finished. Here, the mirror finish refers to, for example, processing the surface so that the height of the unevenness of the surface on which the reflective layer is formed is smaller than the wavelength of light emitted from the active layer. .

  According to the present invention, by providing a nitride semiconductor multilayer structure having a substantially nonpolar surface or a substantially semipolar surface as a main surface, it is possible to emit polarized light in the active layer. Can be increased. Of the emitted light, the light traveling to the opposite side of the light extraction surface is generally scattered by an external housing or the like and the polarization ratio is lowered. In the present invention, however, the reflection made of a Schottky-connected metal is used. By providing a layer, light traveling in the direction opposite to the light extraction surface can be reflected to the light extraction surface, so that light is emitted from the opposite side of the light extraction surface and scattered outside. The resulting decrease in the polarization ratio can be suppressed. Thereby, it is possible to increase the polarization ratio while increasing the amount of light extracted from the light extraction surface.

  Moreover, since the surface on which the reflective layer is formed is mirror-finished, light incident on the reflective layer can be prevented from being scattered by the surface, so that light with a higher polarization ratio can be extracted.

  Hereinafter, a first embodiment in which the present invention is applied to a light emitting diode (LED) will be described with reference to the drawings. FIG. 1 is a cross-sectional view of the semiconductor light emitting device (light emitting diode) according to the first embodiment.

  As shown in FIG. 1, the semiconductor light emitting device 1 includes a substrate 2, a nitride semiconductor multilayer structure 3 laminated on the substrate 2, an anode electrode 4, a cathode electrode 6, and a reflective layer 7. .

  The substrate 2 is made of a single crystal of GaN (gallium nitride). The method for producing a single crystal of GaN is not particularly limited. A nitride semiconductor multilayer structure 3 is laminated on the main surface of the substrate 2. The main surface of the substrate 2 is an m-plane that is a nonpolar surface. The m-plane is a plane (for example, (10-10) plane) corresponding to the side of the hexagonal column when the crystal structure of GaN is approximated to a hexagonal hexagonal crystal. In addition, the bottom surface 2a which is the surface opposite to the light extraction direction A among the surfaces of the substrate 2, that is, the surface on the side where the reflective layer 7 is formed, emits light by the active layer 12 whose surface unevenness is described later. The mirror surface is processed so as to be smaller than the wavelength of the emitted light.

  In the nitride semiconductor multilayer structure 3, an n-type contact layer 11, an active layer 12, a final barrier layer 13, a p-type electron blocking layer 14, and a p-type contact layer 15 are laminated in order from the substrate 2 side. Yes. Here, as described above, since the main surface of the substrate 2 is configured by the m-plane, the main surface of the nitride semiconductor multilayer structure 3 laminated on the main surface of the substrate 2 is also polarized in the active layer 12. The m-plane is a nonpolar plane capable of emitting light.

The n-type contact layer 11 is composed of an n-type GaN layer having a thickness of about 3 μm or more doped with silicon having a concentration of about 1 × 10 ¹⁸ cm ⁻³ as an n-type dopant.

  The active layer 12 has a quantum well structure in which an InGaN layer with a thickness of about 3 nm doped with silicon and a GaN layer with a thickness of about 9 nm are alternately stacked for five periods. This active layer 12 emits light of about 430 nm.

  The final barrier layer 13 is composed of a GaN layer having a thickness of about 40 nm. The doping may be any of p-type, n-type and non-doped, but non-doped is preferred.

The p-type electron blocking layer 14 is an AlGaN layer having a thickness of about 28 nm doped with magnesium having a concentration of about 3 × 10 ¹⁹ cm ⁻³ as a p-type dopant.

The p-type contact layer 15 is made of a GaN layer having a thickness of about 70 nm doped with magnesium having a concentration of about 1 × 10 ²⁰ cm ⁻³ as a p-type dopant. The light extraction side surface 15 a on the light extraction direction A side of the p-type contact layer 15 is for extracting light emitted from the active layer 12 from the nitride semiconductor multilayer structure 3. The surface of the light extraction side surface 15a is mirror-finished so that the unevenness is about 100 nm or less in order to suppress light scattering and suppress a decrease in the polarization ratio. For example, a flat mirror surface as described above can be obtained by crystal growth.

  The anode electrode 4 is made of a metal layer in which a Ni layer and an Au layer are stacked in order from the p-type contact layer 15 side. The anode electrode 4 is ohmically connected to the p-type contact layer 15 and the p-type contact layer 15 in order to allow a current to flow uniformly in the entire region of the nitride semiconductor multilayer structure 3 in the horizontal direction (direction perpendicular to the stacking direction). It is formed so as to cover substantially the entire upper surface. The anode electrode 4 has a thickness of about 200 mm or less capable of transmitting the light emitted from the active layer 12. The light extraction surface 4a of the anode electrode 4 is a surface from which the light emitted by the active layer 12 is extracted, and the surface unevenness is about 100 nm or less, like the light extraction side surface 15a of the p-type contact layer 15. Mirror finish. For example, if the electron beam evaporation method is used, the mirror surface as described above can be obtained. In this way, the light emitted from the active layer 12 is suppressed by the mirror-extracted light extraction side surface 15a and the light extraction surface 4a, so that the light is extracted while maintaining a high polarization ratio. A connection portion 5 in which a Ti layer and an Au layer are stacked is provided in a partial region on the anode electrode 4.

  The cathode electrode 6 is formed by laminating a Ti layer and an Al layer. The cathode electrode 6 is formed in an ohmic connection with the exposed region of the upper surface of the n-type contact layer 11.

  The reflection layer 7 is for reflecting the light traveling in the direction opposite to the light extraction direction A in the light extraction direction A, and is formed on the bottom surface 2a of the substrate 2 which is the surface opposite to the light extraction surface 4a. Is formed. The reflective layer 7 is made of Al, and is Schottky connected to the bottom surface 2a of the substrate 2 to reflect light. In addition, the material which comprises the reflection layer 7 is not limited to Al, You may comprise by Pt, Ag, etc.

  Next, the operation of the semiconductor light emitting device 1 described above will be described. In the semiconductor light emitting device 1, when electrons are supplied from the anode electrode 4 and holes are supplied from the cathode electrode 6, electrons are injected into the active layer 12 through the n-type contact layer 11, and the semiconductor layers 13 to 13 are formed. Holes are injected into the active layer 12 via 15. The electrons and holes injected into the active layer 12 are combined to emit light of about 430 nm. Here, since the main surface of the nitride semiconductor multilayer structure 3 is a non-polar m-plane, the light emitted from the active layer 12 is polarized.

  Of the polarized light, the light traveling in the light extraction direction A passes through the semiconductor layers 13 to 15 and the anode electrode 4 and is extracted outside. On the other hand, the light traveling in the direction opposite to the light extraction direction A is reflected in the light extraction direction A by the reflective layer 7. Here, since the bottom surface 2a of the substrate 2 on the side where the reflective layer 7 is formed is mirror-finished, light scattering can be suppressed at the bottom surface 2a. For this reason, since the fall of the polarization ratio of light can be suppressed, the light which progressed to the board | substrate 2 side is permeate | transmitted through the semiconductor layers 13-15 and the anode electrode 4 with the polarization state substantially maintained, and outside. It is taken out.

  Next, a method for manufacturing the semiconductor light emitting element described above will be described.

  First, a substrate 2 made of a single crystal of GaN having a non-polar m-plane main surface is prepared. Here, the substrate 2 having the m-plane which is a nonpolar plane as the main surface is first cut out of a GaN single crystal substrate having the C-plane as the main surface, and then both the (0001) direction and the (11-20) direction. The surface is polished by a CMP method (chemical mechanical polishing method) so that the azimuth error is within ± 1 ° (preferably within ± 0.3 °), and is mirror-finished. As a result, it is possible to obtain the substrate 2 having the m-plane as a main surface, few crystal defects such as dislocations and stacking faults, and the surface step is suppressed to the atomic level.

  Next, the nitride semiconductor multilayer structure 3 is grown on the above-described substrate 2 by MOCVD. Specifically, first, the substrate 2 is introduced into a processing chamber of an MOCVD apparatus (not shown) and placed on a susceptor that can be heated and rotated. Note that the inside of the processing chamber is set to 1/10 atm to normal pressure, and the atmosphere in the processing chamber is always exhausted.

Next, in order to grow a GaN layer while suppressing surface roughness, the temperature of the substrate 2 is set to about 1000 ° C. while ammonia gas is supplied by a carrier gas (H ₂ gas) into the processing chamber in which the substrate 2 is held. Raise the temperature to about 1100 ° C.

  Next, after the temperature of the substrate 2 rises to about 1000 ° C. to about 1100 ° C., ammonia, trimethyl gallium, and silane are supplied to the processing chamber by a carrier gas, and an n-type GaN layer made of silicon-doped n-type GaN layer is formed. The contact layer 11 is grown.

  Next, after the temperature of the substrate 2 is set to about 700 ° C. to about 800 ° C., ammonia and trimethyl gallium are supplied to the processing chamber by a carrier gas to grow a non-doped GaN layer. An InGaN layer doped with silicon is grown by supplying trimethylindium.

  Then, the active layer 12 having a quantum well structure is formed by alternately repeating the process of growing the non-doped GaN layer and the silicon-doped InGaN layer a desired number of times. Thereafter, ammonia and trimethylgallium are supplied to the processing chamber by the carrier gas, and the final barrier layer 13 made of the GaN layer is grown.

  Next, after raising the temperature of the substrate 2 to about 1000 ° C. to about 1100 ° C., ammonia gas, trimethylgallium, trimethylaluminum and ethylcyclopentadienylmagnesium are supplied to the processing chamber by a carrier gas, A p-type electron blocking layer 14 made of a doped p-type AlGaN layer is grown.

  Next, while maintaining the temperature of the substrate 2 at about 1000 ° C. to about 1100 ° C., ammonia gas, trimethyl gallium and ethylcyclopentadienyl magnesium are supplied to the processing chamber by a carrier gas, and the GaN layer doped with magnesium is supplied. A p-type contact layer 15 made of is grown. Thereby, the nitride semiconductor multilayer structure 3 is completed.

  Next, the reflective layer 7 made of Al is grown on the bottom surface 2a of the substrate 2 by a metal vapor deposition apparatus using an electron beam vapor deposition method or a resistance heating method. Here, in order to connect the reflective layer 7 and the substrate 2 in a Schottky connection, a process such as annealing is not performed after the reflective layer 7 is formed.

  Next, the anode electrode 4 is formed by a resistance heating method or a metal vapor deposition apparatus using an electron beam. Thereafter, the substrate 2 on which the nitride semiconductor multilayer structure 3 and the reflective layer 7 are formed is moved to an etching chamber, and a part of the nitride semiconductor multilayer structure 3 is plasma so that a part of the n-type contact layer 11 is exposed. Etch.

  Next, the connection portion 5 and the cathode electrode 6 are formed by a metal vapor deposition apparatus using a resistance heating method or an electron beam vapor deposition method. Thereafter, each element is divided by cleavage to complete the semiconductor light emitting element 1 shown in FIG.

  Next, an experiment conducted to prove the effect of the semiconductor light emitting device according to the present invention will be described.

  First, as an example, a semiconductor light emitting device was fabricated in which a reflective layer having a thickness of 200 nm was provided on the bottom surface of the substrate, which is the surface opposite to the light extraction surface. Further, in these semiconductor light emitting devices, a polarizing plate in which the polarization direction of the polarized light emitted from the active layer is parallel to the polarization direction is provided as Example A1, and the polarized light emitted by the active layer is polarized. A polarizing plate having a direction perpendicular to the direction of polarization and a polarizing plate provided on the light extraction surface was produced as Example B1.

  Next, as a comparative example, a semiconductor light-emitting element having the same configuration as that of the example was manufactured except that the bottom surface of the substrate did not have a reflective layer. Furthermore, these semiconductor light-emitting elements are provided with a polarizing plate having a polarization direction parallel to the polarization direction of the light emitted by the active layer on the light extraction surface as Comparative Example A2, and polarized light of the polarization emitted by the active layer A polarizing plate having a direction perpendicular to the direction of polarization and a polarizing plate provided on the light extraction surface was produced as Comparative Example B2.

  Next, each emission intensity was measured. The measurement results of the emission intensity at each wavelength in Examples A1 and B1 are shown in FIG. 2, and the measurement results of the emission intensity at each wavelength in Comparative Examples A2 and B2 are shown in FIG.

Here, assuming that the light intensities of Examples A1 and B1 and Comparative Examples A2 and B2 based on the measured emission intensity are I _A1 , I _B1 , I _A2 , and I _B2 , the polarizations of Examples A1 and B1 The ratio ρ ₁ is
ρ ₁ = (I _A1 −I _B1 ) / (I _A1 + I _B1 ) = 0.87
The polarization ratio ρ ₂ of Comparative Examples A2 and B2 is
ρ ₂ = (I _A2 −I _B2 ) / (I _A2 + I _B2 ) = 0.84
It became. As a result, it was proved that the polarization ratio increased by 0.03 by providing the reflective layer.

  As described above, the semiconductor light emitting device 1 according to the first embodiment includes the nitride semiconductor multilayer structure 3 whose main surface is the nonpolar m-plane, so that polarized light is emitted from the active layer 12. . Of the light emitted here, the light traveling in the direction opposite to the light extraction direction A is generally emitted from the bottom surface of the substrate and then scattered by an external housing or the like to lower the polarization ratio. In the semiconductor light emitting device 1 according to the embodiment, by providing the reflective layer 7 on the bottom surface 2a of the substrate 2, light traveling in the direction opposite to the light extraction direction A can be reflected in the light extraction direction A. A decrease in the polarization ratio caused by being emitted from the bottom surface 2a of the substrate 2 and being scattered outside can be suppressed.

  Accordingly, the amount of light extracted from the light extraction surface 4a can be improved, and light having a high polarization ratio can be extracted from the light extraction surface 4a. Furthermore, when the bottom surface 2a of the substrate 2 is mirror-finished, when light enters the reflective layer 7 from the substrate 2, light scattering at the bottom surface 2a can be suppressed, so that light with a higher polarization ratio is extracted. be able to.

  Thus, since the semiconductor light emitting device 1 can extract light having a high polarization ratio, when the semiconductor light emitting device 1 is applied as a light source of a liquid crystal display, one polarizing filter for polarizing the light can be omitted. Or the ratio of the light which permeate | transmits a polarizing filter can be enlarged.

  As mentioned above, although this invention was demonstrated in detail using embodiment, this invention is not limited to embodiment described in this specification. The scope of the present invention is determined by the description of the claims and the scope equivalent to the description of the claims. Hereinafter, modified embodiments in which the above-described embodiment is partially modified will be described.

  For example, the material and thickness constituting each layer, the concentration of the dopant, and the like can be changed as appropriate.

  Moreover, although the example which applied this invention to the light emitting diode was shown in the above-mentioned embodiment, you may apply this invention to other apparatuses, such as a laser.

  In the above-described embodiment, the main surface of the substrate 2 is the m-plane, but the main surface of the substrate is not limited to the m-plane, and it is an abbreviation that can polarize light emitted from the active layer. You may comprise by a nonpolar surface or a substantially semipolar surface. In addition, a non-polar surface or a substantially non-polar surface is not only a non-polar surface and a semi-polar surface, but also a surface having an off angle within ± 1 ° from the non-polar surface and an off within ± 1 ° from the semi-polar surface. It is a concept including a surface having a corner. Here, the crystal structure and crystal plane of GaN constituting the substrate will be briefly described. The crystal structure of GaN can be approximated by a hexagonal hexagonal system. A surface having the C axis along the axial direction of the hexagonal column as a normal is a C surface (0001). As is known, in the GaN crystal structure, since the polarization direction is along the C-axis direction, the C-plane shows different properties between the + C-axis side surface and the −C-axis side surface. (Polar Plane). On the other hand, the side surfaces of the hexagonal columns are m-planes (10-10), respectively, and the plane passing through a pair of ridge lines that are not adjacent to each other is the a-plane (11-20). These are crystal planes perpendicular to the C-plane and orthogonal to the polarization direction, and thus are nonpolar planes having no polarity. Further, the crystal plane that is neither parallel nor perpendicular to the C plane is inclined with respect to the polarization direction, and is therefore a semipolar plane having a slight polarity. Specific examples of the semipolar plane include (10-1-1) plane, (10-1-3) plane, (11-22) plane, (11-24) plane, and (10-12) plane. .

The material constituting the substrate is not limited to a GaN single crystal, but a sapphire substrate whose principal surface is an m-plane or a-plane, a spinel substrate whose principal surface is a (100) plane or (110) plane, An m-plane SiC substrate, LiAlO ₂ substrate, or the like can also be applied.

It is sectional drawing of the semiconductor light-emitting device by 1st Embodiment. It is a figure which shows the relationship between the emitted light intensity by an Example, and a wavelength. It is a figure which shows the relationship between the emitted light intensity by a comparative example, and a wavelength.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor light-emitting device 2 Substrate 2a Bottom surface 3 Nitride semiconductor laminated structure 4 Anode electrode 4a Light extraction surface 5 Connection part 6 Cathode electrode 7 Reflective layer 11 Type contact layer 12 Active layer 13 Final barrier layer 14 n-type electron blocking layer 15 p-type Contact layer 15a Light extraction side

Claims (2)

In a semiconductor light emitting device having a nitride semiconductor multilayer structure including an active layer capable of emitting light,
The main surface of the nitride semiconductor multilayer structure is a substantially nonpolar surface or a substantially semipolar surface,
The surface opposite to the light extraction surface from which the light emitted by the active layer is extracted is provided with a reflection layer for reflecting light to the light extraction surface,
The reflective layer is made of metal and is Schottky connected to the formed surface.   2. The semiconductor light emitting element according to claim 1, wherein the surface on which the reflective layer is formed is mirror-finished.

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